There has been increasing interest in the use of living organisms to control the spread of detrimental insects through agricultural areas. Such insecticidal agents are desirable in order to avoid the drawbacks associated with chemical insecticides, such as their lack of specificity, residual toxic effects and the rapid development of resistance by the targeted insects. Living insecticide agents, when delivered under controlled conditions, have narrow host ranges and can control the spread of specific hosts, without affecting natural predators or beneficial insects. Examples of such agents, termed bio-rational insecticides, include Bacillus thuringiensis: Baculoviridae, such as Autographa californica nuclear polyhedrosis virus; and various fungal pathogens, among others.
Nematodes have long been considered a desirable insecticide agent due, in part, to their wide variety of target or host organisms. For example, steinernematids and heterorhabditid nematodes display a broad host range under laboratory conditions which exclude behaviorial or ecological barriers to nematode infection, Poinar, G. O., Nematodes for Biological Control of Insects, CRC Press, Inc., Boca Ratan, Florida (1979); Gaugler, R., J. Nematol. 13:241-249 (1981). The insecticidal effect generally results from the nematodes own pathogenicity towards insects, as well as its association with certain entomogenous bacteria. For example, the infective larvae of Neoaplectana carpocapsae have an associated bacteria Achromobacter nematophilus, usually found in the intestinal lumen. Following ingestion by an insect, or invasion of the insect, the nematode will usually penetrate the gut wall and enter the hemocoel, whereupon the bacteria will be released and multiply to produce fatal septicaemia in the host.
A major obstacle to the use of nematodes as insecticides has been their susceptibility to desiccation. In the field, the effective host range is limited, by the nematodes moisture requirement, to insects inhabiting the soil and cryptic habitats including, e.g., greenhouses, mushroom beds and animal dung. There have been numerous attempts to increase field persistance and utilize nematodes for control of insects in the open by avoiding diurnal application or employing anti-desiccants or humectants.
This moisture requirement has led to various aqueous formations containing nematodes, whose effectiveness is generally limited by premature evaporation of the aqueous carrier. In order to reduce the rate of evaporation, aqueous carriers have included evaporation-retardant water thickeners, mineral oil, gelling agents or surfactants. See, for example, U.S. Pat. No. 4,178,366.
Hydrogel agents have been employed for the encapsulation of numerous microorganisms or cell cultures as well as organic and bio-active chemicals. Representative of such encapsulations are U.S. Pat. Nos. 4,450,233; 4,352,883; 4,434,231.
It would be desirable to encapsulate multi-cellular organisms, such as nematodes, in a matrix which supplies sufficient moisture to prevent desiccation, yet allows the infective stage of the nematode to be ingested by or invade a broad range of insect hosts.